ACADs (08-006) Covered Keywords

1. Interactions with Matter ACADs (08-006) Covered Keywords Energy transfer, ionization, excitation, Bremsstrahlung, linear energy transfer, alpha, beta, gamma, range, Compton scattering, indirectly ionizing, pair production, neutron interactions, fission, elastic scattering, inelastic scattering, radiation shielding. Description Supporting Material

2. Radiation’s Interactionwith Matter

3. Energy Transfer Mechanisms • Introduction • All radiation possesses energy • Inherent — electromagnetic • Kinetic — particulate • Interaction results in some or all of the energy being transferred to the surrounding medium • Scattering • Absorption

4. Energy Transfer Mechanisms • Ionization • Removing bound electron from an electrically neutral atom or molecule by adding sufficient energy to the electron, allowing it to overcome its BE • Atom has net positive charge • Creates ion pair consisting of negatively charged electron and positively charged atom or molecule

5. Energy Transfer Mechanisms Ionizing Particle e- e- Negative Ion N P+ P+ Positive Ion N e-

6. Energy Transfer Mechanisms • Excitation • Process that adds sufficient energy to e- or molecule such that it occupies a higher energy state than it’s lowest bound energy state • Electron remains bound to atom or molecule, but depending on role in bonds of the molecule, molecular break-up may occur • No ions produced, atom remains neutral

7. Energy Transfer Mechanisms • After excitation, excited atom eventually loses excess energy when e- in higher energy shell falls into lower energy vacancy • Excess energy liberated as X-ray, which may escape from the material, but usually undergoes other absorptive processes

8. P+ N N P+ Energy Transfer Mechanisms e- e- e- N e- + + N N N + + e- N e- e-

9. Energy Transfer Mechanisms • Bremsstrahlung • Radiative energy loss of moving charged particle as it interacts with matter through which it is moving • Results from interaction of high-speed, charged particle with nucleus of atom via electric force field • With negatively charged electron, attractive force slows it down, deflecting from original path

10. Energy Transfer Mechanisms • KE particle loses emitted as x-ray • Production enhanced with high-Z materials (larger coulomb forces) and high-energy e- (more interactions occur before all energy is lost)

11. Energy Transfer Mechanisms e- e- e- e- N e- + + N N N + + e- N e- e-

12. Directly Ionizing Radiation • Charged particles don’t need physical contact with atom to interact • Coulombic forces will act over a distance to cause ionization and excitation • Strength of these forces depends on: • Particle energy (speed) • Particle charge • Absorber density and atomic number

13. Directly Ionizing Radiation • Coulombic forces significant over distances > atomic dimensions • For all but very low physical density materials, loss of KE for e- continuous because of “Coulomb force” • As charged particle passes through absorber, energy loss can be measured several ways

14. Directly Ionizing Radiation • Specific Ionization • Number of ion pairs formed per unit path length • Often used when energy loss is continuous and constant, such as with α or β- particles • Number of pairs produced depends on ionizing particle type and material being ionized • α — 80,000 ion pairs per cm travel in air • β — 5,000 ion pairs per cm travel in air

15. Directly Ionizing Radiation • Linear Energy Transfer • Average energy a charged particle deposits in an absorber per unit distance of travel (e.g., keV/cm) • Used to determine quality factors for calculating dose equivalence

16. Directly Ionizing Radiation • Alpha Interactions • Mass approximately 8K times > electron • Travels approximately 1/20th speed of light • Because of mass, charge, and speed, has high probability of interaction • Does not require particles touching—just sufficiently close for Coulombic forces to interact

17. Directly Ionizing Radiation • Energy gradually dissipated until α captures two e- and becomes a He atom • α from given nuclide emitted with same energy, consequently will have approximately same range in a given material

18. Directly Ionizing Radiation • Calculating Range • Approximate general formula for range in air in cm for E < 4 MeV for 4 < E < 8 MeV Where: E = Alpha energy

19. Directly Ionizing Radiation • In other media, range in media (Rm in mg/cm2) Where: A = atomic mass of absorber

20. Directly Ionizing Radiation Sample Problem How far will a 2.75 MeV α travel in air? How far will that same α travel in ?

21. Directly Ionizing Radiation Sample Problem How far will a 950 keV α travel in air? How far will it travel in ?

22. Directly Ionizing Radiation What is a beta? An unbound electron with KE. It’s rest mass and charge are the same as that of an orbital electron.

23. Directly Ionizing Radiation • Beta Interactions • Interaction between β- or β+ and an orbital e- is interaction between 2 charged particles of similar mass • βs of either charge lose energy in large number of ionization and/or excitation events, similar to α • Due to smaller size/charge, lower probability of interaction in given medium; consequently, range is >> α of comparable energy

24. Directly Ionizing Radiation • Because β’s mass is small compared with that of nucleus • Large deflections can occur, particularly when low-energy βs scattered by high-Z elements (high positive charge on the nucleus) • Consequently, β usually travels tortuous, winding path in an absorbing medium • β may have Bremsstrahlung interaction resulting in X-rays

25. Directly Ionizing Radiation • Calculating Range (mg/cm2) for 0.01 < E < 2.5 MeV for E > 2.5 MeV Where: E = Beta energy

26. Directly Ionizing Radiation Sample Problem How far will a 90Sr β- travel in air?

27. Directly Ionizing Radiation Sample Problem How far will a 133Xe β- travel in air?

28. Indirectly Ionizing Radiation • No charge • γand n • No Coulomb force field • Must come sufficiently close for physical dimensions to contact particles to interact • Small probability of interacting with matter

29. Indirectly Ionizing Radiation • Don’t continuously lose energy by constantly interacting with absorber • May move “through” many atoms or molecules before contacting electron or nucleus • Probability of interaction depends on its energy and absorber’s density and atomic number • When interactions occur, produces directly ionizing particles that cause secondary ionizations

30. Indirectly Ionizing Radiation • Gamma absorption • γ and x-rays differ only in origin • Name used to indicate different source • γs originate in nucleus • X-rays are extra-nuclear (electron cloud) • Both have 0 rest mass, 0 net electrical charge, and travel at speed of light • Both lose energy by interacting with matter via one of three major mechanisms

31. Indirectly Ionizing Radiation • Photoelectric Effect • All energy is lost – happens or doesn’t • Photon imparts all its energy to orbital e- • Because pure energy, photon vanishes • Probable only for photon energies < 1 MeV • Energy imparted to orbital e- in form of KE, overcoming attractive force of nucleus, usually causing e- to leave orbit with great velocity

32. Indirectly Ionizing Radiation • High-velocity e-, called photoelectron • Directly ionizing particle • Typically has sufficient energy to cause secondary ionizations • Most photoelectrons inner-shell (K) electrons

33. Indirectly Ionizing Radiation • Probability of interaction per gram of absorber (σ) • Directly proportional to cube of atomic number (Z) • Inversely proportional to cube of photon energy (E) Where: Z = Absorber atomic number E = Photon energy

34. Indirectly Ionizing Radiation • When calculating σ for absorber with >1 element, must use Zeff • a1 and a2 are the fraction of total electrons in the compound • For water (H2O) = 10 electrons total • H = 2/10 = 0.2 • O = 8/10 = 0.8

35. Indirectly Ionizing Radiation Sample Problem What is the probability of a 661 keV photon interacting with a water molecule?

36. Indirectly Ionizing Radiation Sample Problem cont’d

37. Indirectly Ionizing Radiation Sample Problem What is the probability of a 2.4 MeV photon interacting with a molecule of BF3?

38. Indirectly Ionizing Radiation e- Photoelectron Gamma Photon (< 1 MeV) e- e- N e- + + N N N + + e- N e- e-

39. Indirectly Ionizing Radiation • Compton Scattering • Partial energy loss for the incoming photon • Dominant interaction for most materials for photon energies 200 keV-5 MeV • Photon continues with less energy in different direction to conserve momentum • Probability of Compton interaction  with distance from nucleus — most Compton electrons are valence electrons

40. Indirectly Ionizing Radiation • Beam of photons may be randomized in direction and energy, so that scattered radiation may appear around corners and behind shields where there is no direct line of sight to the source • Probability of Compton interaction  with distance from nucleus — most Compton electrons are valence electrons • Difficult to represent probability mathematically

41. Indirectly Ionizing Radiation e- Compton Electron Gamma Photon (200 keV < E < 5 MeV) e- e- e- N Reduced energy (scattered photon) + + N N N + + e- N e- e-

42. Indirectly Ionizing Radiation • Pair Production • Occurs when all photon energy is converted to mass (occurs only in presence of strong electric field, which can be viewed as catalyst) • Strong electric fields found near nucleus and are stronger for high-Z materials • γ disappears in vicinity of nucleus and β-- β+ pair appears

43. Indirectly Ionizing Radiation • Will not occur unless γ> 1.022 MeV • Any energy > 1.022 MeV shared between the β--β+ pair as KE • Probability < photoelectric and Compton interactions because photon must be close to the nucleus

44. Indirectly Ionizing Radiation Electron e- Gamma Photon (E > 1.022 MeV) e- e- e+ N e- + + Positron N N N + + e- e- N 0.511 MeV Photons e- e-

45. Absorption and Attenuation • To describe energy loss and photon beam intensity penetrating absorber, need to know absorption and attenuation coefficient of absorbing medium. • Represents probability or cross-section for interaction. • Represented by µ

46. Absorption and Attenuation • Represents the sum of the individual probabilities • Because each reaction is energy dependent, µ is energy dependent too

47. Absorption and Attenuation • Curve has three distinct regions • Curve makes more sense when individual contributions are shown

48. Absorption and Attenuation • Dividing line between low and medium occurs at energy where PE and CS are equally likely • Intersection of two dashed lines

49. Absorption and Attenuation • Energy dependent • Tissue — 25 keV • Al and Bone — 50 keV • Pb— 700 keV

50. Absorption and Attenuation • µ = fraction of photons in beam that interact per unit distance of travel • If expressed in per cm units (cm-1), then numerically equal to fraction of interactions, by any process, in an absorber of 1 cm thickness • µ depends on number of electrons in path, so is proportional to density • While the same substance, ice, water, and steam all have different µs at any given energy